This invention relates to a lockup control apparatus for use with an automotive vehicle having an engine, an automatic transmission and a torque converter having a lockup clutch operable on a variable engagement force to provide a controlled degree of mechanical connection between the engine and the automatic transmission.
Various techniques for automatic transmission lockup control have been proposed. Assuming now that the target lockup engagement force value is decreased rapidly in a stepped fashion to its minimum value required during deceleration, as shown in FIG. 9A, to reduce the lockup engagement force, as shown in FIG. 9B, the response is improved when the lockup condition is released after the lockup engagement force reduction because of decreased deformation of the lockup clutch and the torque converter cover. In this case, the degree of reduction of the engine speed decreases as the lockup engagement force decreases. For this reason, it is possible to release the lockup condition at a sufficient response rate by controlling the lockup engagement force to its minimum value during deceleration. Therefore, the engine can be prevented from stalling in the presence of sudden braking (at time t1) upon a change in a lockup condition from an acceleration condition to a deceleration condition. When the lockup engagement force is decreased just after the throttle valve becomes OFF (the accelerator pedal is released), however, the lockup engagement force decreases at a greater rate than the engine output. As a result, the lockup engagement force becomes so insufficient as to cause slip rotation, as indicated by the broken curve of FIG. 9C, to obstruct smooth output shaft torque reduction.
The reason for the delay of the engine output reduction with respect to the lockup engagement force reduction is as follows: It is the current practice to delay the termination of delivery of fuel to the engine or correct the intake air flow rate so as to prevent a rapid drop in the engine output (rapid engine output drop preventive control) just after the throttle valve becomes OFF, as indicative by the broken curve of FIG. 9D, in order to avoid vibrations and a sense of rapid deceleration to the driver which may be caused a rapid engine output drop, as indicated by the solid curve of FIG. 9B, when the throttle position decreases at a rapid rate, as indicated by the solid curve of FIG. 9D. Thus, the lockup engagement force cannot be decreased at a rapid rate during the rapid engine output drop preventive control without providing a sense of incompatibility to the driver. If the brake pedal is depressed immediately after the condition that the throttle valve is in its OFF position in a short time during the rapid engine output drop preventive control, the engine would stall. Furthermore, if the lockup engagement force is decreased at a rapid rate, the lockup control fluid pressure will overshoot to cause slip rotation or failure to achieve lockup engagement during deceleration in spite of the fact that a target lockup engagement force value sufficient to hold the lockup engagement condition is commanded.